A complaint has been filed to the European Commission accusing Microsoft of stifling competition by tying its browser to Windows.

Opera Software said the close ties between Internet Explorer (IE) and Windows made it hard for rivals to be a serious choice for web users.

Opera also said Microsoft flouted web standards, making it much harder for browsers to be interoperable.

Microsoft said putting its browser in its operating system benefited users.

Unfair advantage

The EC confirmed that it had received the complaint from the Norwegian software firm and said it would be studied carefully.

The complaint said the bundling of IE with Windows gave the software giant an unfair advantage and made competition much more difficult.

In a statement Opera said it wanted the Commission to make Microsoft separate IE from Windows and pre-install alternative browsers on new PCs.

It also wants Microsoft to be forced to follow web specifications rather than its own "de facto" standards.

"Our complaint is necessary to get Microsoft to amend its practices," said Jason Hoida, Opera Software's deputy general counsel.

'Free to choose'

In response Microsoft said the company would co-operate with any enquiries and added: "We believe the inclusion of the browser into the operating system benefits consumers, and that consumers and PC manufacturers already are free to choose any browsers they wish."

It added that the IE browser had been part of Windows for more than a decade.

The dominance of Microsoft's Internet Explorer browser has been dented in recent years by the success of other net surfing programs such as Firefox.

Estimates vary but IE is thought to be used by approximately 80% of web users. Opera is believed to have a 1-2% market share of web users.

The complaint comes after Microsoft lost a long-running competition dispute with the European Commission.

After its final appeal against the ruling over its bundling of its media player with Windows, Microsoft had to pay a 497m euro (£343m; $690m) fine.